The present invention relates to methods and apparatus for purification of water by filtration. More specifically, it concerns improved water filtration systems with an upflow filter which contains a buoyant filter media in series with a downstream downflow filter which contains a nonbuoyant filter media. In addition, the invention also concerns an improved apparatus for cleaning such a system.
A prior art water filtration system having an upflow filter with a buoyant filter media followed by a downflow filter with a nonbuoyant filter media is disclosed in U.S. Pat. No. 4,547,286 of Hsiung. In one embodiment of this system, the buoyant media filter and nonbuoyant media filter are separated by a compartment or well. During normal operation of the system, pretreated water from the buoyant media filter flows through the well and to the nonbuoyant media filter. To clean the nonbuoyant media filter, the water level within the well is lowered below the level of the inlet to the nonbuoyant media filter. Air and flush water is then passed upwardly through the filter and to the well. Solids dislodged from the buoyant media filter are carried into the well and withdrawn without reaching the nonbuoyant media filter. In addition, the water level in the well is lowered, and the nonbuoyant media filter is backwashed into the well during cleaning.
This prior art system is capable of producing high quality filtered water at flow rates on the order of 10 gallons per minute per square foot of filter area. Also, in most applications, this system eliminated the need for a pretreatment settling tank or clarifier. Instead, chemicals, such as coagulants, are injected directly into the raw water stream upstream of the buoyant media filter. Flocculation and clarification takes place in this first filter stage. Also, this system monitors water conditions, such as turbidity of the influent or effluent. The result of this monitoring is used to establish and control chemical dosages to minimize the waste of chemicals. Also, chemical savings were realized over systems utilizing a separate settling tank clarifier.
In addition, in this prior art system, partial cleaning of the buoyant media filter with less frequent full cleaning of the nonbuoyant media filter is utilized. Cleaning is accomplished after a predetermined time, or after a predetermined head loss is sensed across the media bed. Solids remaining in the buoyant media filter following partial cleaning enhance flocculation of material from influent raw water, resulting in additional saving of pretreatment chemicals.
Although this prior art water filtration system offers a number of advantages, inefficiencies exist therein, particularly in its cleaning mechanism. Therefore, a need exists for an improved water filtration system having an upflow buoyant media filter and a downflow nonbuoyant media filter.